Method of desulfurizing a hydrocarbon

ABSTRACT

A hydrocarbon gas such as methane and LPG is desulfurized in the presence of oxygen and an oxidation catalyst to convert sulfur compounds in the gas to sulfur oxides. The sulfur oxides are then trapped downstream of the oxidation by an adsorbent. The amount of oxygen added to the hydrocarbon gas to promote oxidation is such that the sulfur compounds are selectively oxidized and the oxidation of the hydrocarbon gas is minimized to reduce hydrogen formation.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an improved system for reducingsulfur compound content in a hydrocarbon gas stream. More particularly,the desulfurizing system described herein may be used to reduce thepresence of inorganic and organic sulfur compounds in the gas stream tolevels acceptable for, e.g., subsequent reforming of the hydrocarbon.

[0003] 2. Discussion of the Prior Art

[0004] The partial oxidation of hydrocarbons, for example, methane, inthe presence of a catalyst is an attractive route for the preparation ofmixtures of carbon monoxide and hydrogen, known in the art as synthesisgas. The partial oxidation of a hydrocarbon is an exothermic reactionand, in the case in which methane is the hydrocarbon, proceeds by thefollowing reaction (1):

2CH₄+O₂→2CO+4H₂  (1)

[0005] Another important source of hydrogen and synthesis gas is derivedfrom steam reforming a hydrocarbon such as methane. Catalyzed steamreforming is endothermic and in the case in which methane is thehydrocarbon, proceeds by the following reaction (2):

CH₄+H₂O→CO+3H₂  (2)

[0006] It is well known and desirable to reduce the level of gaseoussulfur compounds such as hydrogen sulfide (H₂S), carbonylsulfide (COS),mercaptans (R—SH), and sulfides (R¹—S—R²) from hydrocarbon streams priorto reforming the hydrocarbon stream into useful gaseous components suchas by either of reactions (I) and (2) above. Many applications, e.g.,fuel cells, require that the gaseous sulfur compounds in a gas stream(e.g., naphtha, liquid petroleum gas (LPG), town gas, etc.) be reducedto as low a level as practicable in order to avoid polluting theenvironment or poisoning (i.e., deactivating) catalysts such as steamreforming catalysts, water-gas shift catalysts, etc. Fuels, such asnatural gas, gasoline, diesel fuel, naphtha, fuel oil, LPG and likehydrocarbon fuels may not be useful as a process fuel source due to theexistence of relatively high levels of naturally-occurring complexorganic sulfur compounds, or sulfur compounds added as odorants, such asmercaptans and sulfides.

[0007] Desulfurization of hydrocarbon streams is particularly beneficialfor hydrogen generation and use thereof in fuel cells. Conventional fuelprocessing systems used with stationary fuel cell power plants include athermal steam reformer, such as that described in U.S. Pat. No.5,516,344. In such a fuel processing system, sulfur is removed byconventional hydrodesulfurization techniques, which typically rely on acertain level of recycle as a source of hydrogen for the process. Therecycle hydrogen combines with the organic sulfur compounds to formhydrogen sulfide within a catalytic bed. The hydrogen sulfide is thenremoved using a zinc oxide bed to form zinc sulfide. A generalhydrodesulfurization process is disclosed in detail in U.S. Pat. No.5,292,428. There are many such prior art processes involvinghydrogenation desulfurization in which the sulfur compounds in the fuelstream are decomposed by hydrogenolysis at temperatures of, e.g., 350 to400° C. in the presence of e.g., Ni—Mo or Co—Mo catalysts and thereafterthe resultant H₂S is then absorbed on a bed of ZnO at temperatures of,e.g., 300 to 400° C. However, in these processes, the level of the H₂Sremaining in the treated stream is often too high, e.g., 1 ppmV andhigher. It is well known low levels of gaseous sulfur compounds willdeactivate steam reforming nickel-based catalysts. Additionally, toremove the sulfur compounds from the gas being treated, hydrogen must beprovided to the gas stream. In the case where the source of hydrogen isproduct gas in the form of recycle, this will reduce the overallefficiency of the power forming process.

[0008] Hydrogen sulfide has also been removed from gas streams bypassing the gas stream through a liquid scrubber, such as sodiumhydroxide, potassium hydroxide, or amines. Liquid scrubbers are largeand heavy, and require large chemical inventories. Clean-up of theproduct gas is often needed to prevent carryover of the base scrubbingchemicals.

[0009] Still another process for removing sulfur compounds fromhydrocarbon gas streams involves passing the gas stream directly throughan adsorbent, which captures the sulfur species. Although the adsorptionprocess operates at moderate temperatures and atmospheric pressure, alarge inventory of adsorbent is needed. For natural gas, large volumesof one or more adsorbents are required for reasonable time on stream,e.g., one year, typically up to 20 liters total volume for a 2.5kilowatt electric (kWe) average output fuel cell. Furthermore, naturalgas composition variability makes choosing the appropriate adsorbentsand bed sizes complicated and costly. For LPG, desulfurization byadsorption is particularly difficult due to the potentially high sulfurconcentrations in LPG and adsorption interferences from LPGhydrocarbons.

SUMMARY OF THE INVENTION

[0010] The present invention provides a process for the removal ofsulfur compounds from a hydrocarbon feedstock. The process generallyinvolves passing a hydrocarbon feed with a sub-stoichiometric amount ofan oxygen-containing gas over an oxidation catalyst such that at least aportion of the sulfur compounds in the hydrocarbon feed are converted toSOx without substantial oxidation of the hydrocarbons. An adsorbentplaced downstream of the oxidation catalyst captures and removes theSO_(x) compounds from the hydrocarbon feed. Once desulfurized, thehydrocarbon feed may be reformed to desired products such as hydrogen.

DETAILED DESCRIPTION OF THE INVENTION

[0011] The process detailed herein may be used to desulfurize anygaseous hydrocarbon containing gaseous sulfur compounds, includingvaporized liquid hydrocarbons. The process is particularly suitable forthe desulfurization of methane, natural gas, associated gas or othersources of light hydrocarbons, including LPG. In this respect, the term“light hydrocarbons” is a reference to hydrocarbons having from 1 to 5carbon atoms. The process may be applied in the conversion of naturallyoccurring reserves of methane, which can contain sulfur concentrationson the order of 20 ppmV in the form of organic sulfur compounds, such asmercaptans and sulfides, and inorganic sulfur compounds, such ashydrogen sulfide, carbonyl sulfide, and carbon disulfide. An LPG feedcan contain up to 200 ppmV of these sulfur compounds. The sulfur contentof the hydrocarbon feed should be less than 100 ppbV, preferably lessthan 10 ppbV, to avoid poisoning of the reforming catalyst and adverselyaffecting end uses of the synthesis gas produced.

[0012] In accordance with one embodiment of the process, oxygen (O₂) isintroduced into a hydrocarbon feedstock that is to be desulfurized, andthe mixture is contacted with an oxidation catalyst. Air may be used asthe oxygen source. An O₂/C ratio is established so as to favor theoxidation of the sulfur compounds to SO_(x) when the oxidation catalystis contacted. For example, in the case of the hydrocarbon methane, asthe O₂/C ratio approaches stoichiometric levels with respect to methanepartial oxidation reaction (1), H₂S formation begins to supplant SOxformation due to reaction of the sulfur compounds with hydrogen.Therefore, the oxygen concentration relative to the hydrocarbon feed iscontrolled to limit reaction (1) and favor the selective oxidation ofthe sulfur compounds to SO_(x).

[0013] The gaseous mixture contacted with the catalyst in the process ofthis invention typically comprises a plurality of differentsulfur-containing compounds. Both organic and inorganicsulfur-containing compounds may be present. Examples of inorganic sulfurcompounds that may be present include hydrogen sulfide, carbonylsulfide, and carbon disulfide. Organic sulfur-containing compounds suchas mercaptans and sulfides may also be contained in the hydrocarbonstream being treated.

[0014] The content of sulfur-containing compounds in the hydrocarbonfeed can vary widely, and is typically in the range of from 0.05 to 170ppmV in terms of sulfur (S) content. For natural gas, a sulfur contentof 0.1 to 10 ppmV is more typical. For LPG, a sulfur compound content offrom 10 to 170 ppmV is more typical. Hydrocarbon feed stocks obtaineddirectly from naturally occurring reservoirs may have a sulfur compoundcontent significantly above the aforementioned upper limits and willalso benefit from the sulfur removal treatment described herein.

[0015] The hydrocarbon feedstock, oxygen and its associated gases andthe sulfur-containing compounds are preferably well mixed prior to beingcontacted with the catalyst.

[0016] The oxygen-containing gas, preferably air, is added and mixedwith the hydrocarbon feed in an amount sufficient to establish asuitable oxygen-to-carbon ratio to provide the selective oxidation ofsulfur compounds and minimize the partial oxidation of hydrocarbons tohydrogen. In this process, the sulfur compounds are preferentiallyconverted to SO_(x), which can be readily captured downstream by anadsorbent. The addition of excess air leads to oxidation of thehydrocarbons and formation of hydrogen, which in turn leads to formationof hydrogen sulfide, which can be captured by zinc oxide or othersuitable adsorbent. Thus, depending on the oxygen to carbon ratio, bothSOx and H₂S species may be present in the gas exiting the catalyst andthen trapped downstream using a combination of SOx and H₂S adsorbents.

[0017] The oxygen content of the hydrocarbon stream is characterized assub-stoichiometric. What this means is that the ratio of the molecularoxygen (O₂) content relative to the carbon (C) atoms present in thehydrocarbon feedstock is less than that required for completion ofreaction (1), i.e., less than 0.5. Preferably, the oxygen-to-carbonratio is less than 0.3, more preferably from 0.01 to 0.08. There is agradual increase in the formation of H₂S at O₂/C ratios above 0.04.Therefore, most preferably, the O₂/C ratio should not exceed 0.04. Ifthe O₂/C ratio in the hydrocarbon feed is within the substoichiometricrange contemplated herein without the addition of air or some otheroxygen source, no additional oxygen need be introduced, though oxygenmay still be added to establish a higher O₂/C ratio within thecontemplated range.

[0018] The process may be effectively operated at ambient pressure.However, since some hydrocarbon gases, in particular, vaporized liquidgases are supplied at elevated pressures, the process of the presentinvention may be operated at elevated pressures, that is pressuressignificantly above atmospheric pressure. Thus, the process may beoperated at the vapor pressure of LPG.

[0019] The process may be operated at any suitable temperature. However,under the conditions of pressure prevailing in the process, it isnecessary to allow the feed gases to contact the catalyst at elevatedtemperatures in order to achieve the level of conversion required for acommercial scale operation. Accordingly, the process is preferablyoperated at a temperature of at least 200° C. Preferably, the operatingtemperature is in the range of from 200° C. to 600° C., more preferablyin the range of from 250° C. to 400° C. Temperatures in the range offrom 275° C. to 375° C. are particularly suitable.

[0020] The feed mixture may be provided during the process at anysuitable gas space velocity. It is an advantage of the process of thepresent invention that high gas space velocities may be applied. Thus,typical space velocities for the process are in the range of from 1,000to 50,000/hr, more preferably in the range of from 5,000 to 20,000/hr.

[0021] Catalyst compositions suitable for use in the process are notparticularly limited, so long as the composition can catalyze oxidationof sulfur compounds contained in the hydrocarbon feed to SO_(x) underthe prevailing reaction conditions. Preferred oxidation catalystsinclude, as the catalytically active component, a metal selected fromGroup VIII of the Periodic Table of the Elements, and or base metaloxides such as oxides of chromium, manganese, iron, cobalt, nickel,copper and zinc. More preferred catalysts for use in the processcomprise a metal selected from palladium, platinum and rhodium, and/orbase metal oxides such as oxides of iron, cobalt, and copper. Otherknown oxidation catalysts may also be used, such as vanadium oxides andcerium oxides.

[0022] The catalytically active metal is most suitably supported on acarrier. Suitable carrier materials are well known in the art andinclude the refractory oxides, such as silica, alumina, titania,zirconia, tungsten oxides, and mixtures thereof. Mixed refractoryoxides, that is refractory oxides comprising at least two cations, mayalso be employed as carrier materials for the catalyst.

[0023] The catalytically active metal may be deposited on the carrier bytechniques well known in the art. A most suitable technique fordepositing the metal on the carrier is impregnation, which techniquetypically comprises contacting the carrier material with a solution of acompound of the catalytically active metal, followed by drying andcalcining the resulting material.

[0024] The catalyst (catalytically active material and support) ispreferably sulfur tolerant. For the purposes of this description,“sulfur tolerant” means the catalyst continues to operate-in-thepresence of sulfur.

[0025] The catalyst may comprise the catalytically active metal in anysuitable amount to achieve the required level of activity. Typically,the catalyst comprises the active metal in an amount in the range offrom 0.01 to 20% by weight, preferably from 0.02 to 10% by weight, morepreferably from 0.1 to 7.5% by weight.

[0026] Any suitable reaction regime may be applied in the process inorder to establish contact between the reactants and the catalyst. Onesuitable regime is a fluidized bed, in which the catalyst is employed inthe form of particles fluidized by a stream of gas. A preferred reactionregime for use in the process is a fixed bed reaction regime, in whichthe catalyst is retained within a reaction zone in a fixed arrangement.Pellets of catalyst may be employed in the fixed bed regime, retainedusing fixed bed reaction techniques well known in the art.Alternatively, the fixed arrangement may comprise the catalyst in theform of a monolithic structure. Suitable monolithic structures includerefractory oxide monoliths and ceramic foams.

[0027] Subsequent to oxidation of sulfur compounds in the hydrocarbongas stream to SO_(x) products, these sulfur oxides are removed from thehydrocarbon stream. This is accomplished by placing an adsorbent trapdownstream from the oxidation process whereby the SO_(x) productscontact with the adsorbent to trap and remove the sulfur oxides from thehydrocarbon stream. While the adsorbent material is not particularlylimited so long as it is capable of adsorbing SO_(x) at the prevailingconditions, the sulfur oxide traps preferably comprise alkali metaloxides, alkali earth metal oxides and/or base metal (Fe, Ni, Cu, Zn)oxides, which oxides are preferably supported on porous materials suchas silica, alumina, etc. Under conditions where H₂S is formed, the trapmay further comprise and any effective H₂S adsorbing material, such aszinc oxide. The form of the SOx and H₂S adsorbent materials is notparticularly limited. Preferred forms include pellets and washcoatedmonolithic structures.

[0028] In a preferred embodiment, the desulfurized hydrocarbon stream isreformed into carbon monoxide and/or hydrogen as, for example, by eitheror both reactions (1) or (2) above. A pure hydrogen stream hasparticular use in fuel cells for the generation of electricity. Adiscussion of generating hydrogen for fuel cell operation is given in“The generation of hydrogen for the solid polymer membrane fuel cell”,Robert J. Farrauto, Engelhard Corp., Mar. 13, 2000 and presented at theC.R. Acad. Sci. Paris, Serie llc, Chimie/Chemistry 3 (2000) 573-575.

[0029] The principle of operation of a fuel cell is simple. Hydrogen gasis electrocatalytically oxidized to hydrogen ions at the anode composedof Pt deposited on a conductive carbon. The protons pass through amembrane of a fluoropolymer of sulfonic acid called a proton exchangemembrane. At the Pt on carbon cathode, O₂ from air iselectrocatalytically reduced and combines with the protons producingH₂O. The electrons flow through the external circuit. The cells arestacked in series to generate higher voltages.

[0030] The mixture of carbon monoxide and hydrogen prepared byreformation of the treated hydrocarbon is particularly suitable for usein the synthesis of hydrocarbons, for example by means of theFischer-Tropsch synthesis, or the synthesis of oxygenates, for examplemethanol. Processes for the conversion of the mixture of carbon monoxideand hydrogen into such products are well known in the art.

EXAMPLE 1

[0031] Gas Chromatography (GC) is used to analyze the inlet and outletto a partial oxidation catalyst. The inlet gas contains methane and 16ppmV each of COS, ethylmercaptan, dimethylsulfide, andtetrahydrothiophene. The catalyst is platinum supported on 20% ZrO₂impregnated SiO₂ (Pt/20% ZrO₂—SiO₂) catalyst (80 g/ft³ Pt). Air is addedto the gas such that the O₂ to carbon ratio in the feed is 0.02. Themixture of feed gas and air is passed over the catalyst at 275° C. inlettemperature and 20,000/hr space velocity. The pressure is ambient (1atmosphere). GC analysis (FIG. 1) of the outlet to the catalyst showsthat the organic sulfur compounds in the feed are all converted to aninorganic sulfur compound. GC-MS analysis further confirms that SO₂ theonly sulfur compound formed.

EXAMPLE 2

[0032] In this example, the same methane feed containing the sameconcentration of sulfur compounds as in Example 1 is provided with airat the same O₂/C ratio and treated with the same catalyst and conditionsas in Example 1. A 30% Cs/alumina trap is placed downstream of thecatalyst. The trap is supported on a monolith with a loading of 1 g/in³,and operated at 3000/hr space velocity. The outlet temperature of thetrap is measured at 370° C. No sulfur was observed at the outlet of theCs trap (<10 ppb S) until breakthrough is observed after 26 hours. TheCs/alumina trap has a trapping capacity for SO₂ of 2.7 g S per gCs/alumina trap.

EXAMPLE 3

[0033] In this example, desulfurization of methane is achieved with thesame oxidation catalyst as used in Example 1. 5.25 ppmV each of COS,ethylmercaptan, dimethylsulfide and tetrahydrothiophene were included inthe methane feed stream. Inlet temperature, space velocity and the O₂/Cratio were the same as in Example 1. A downstream trap of 20% K/aluminahaving a trap capacity of 5.9 g S/100 g trap was used to capture theoxidized sulfur species. The test was run for the first 35 hours at aspace velocity for the catalyst of 20,000/hr. For the last hour the timeon stream, the catalyst was operated at 10,000/hr. Sulfur is notdetected at the exit of the trap until the trap capacity is expendedafter 36 hours on stream.

EXAMPLE 4

[0034] This example illustrates the conversion and interconversion ofsulfur compounds at high concentrations of organic sulfur in amethane-propane mixture (Table 1). Natural gas is composed of methaneand lower concentrations of heavier hydrocarbons such as ethane,propane, butane, etc. In this example, methane is combined with propane,carbon dioxide, and air to give a composition 81% methane, 3% propane,4% CO₂, 8.6% air. The resulting O₂/C ratio is 0.02. Sulfur compounds areadded to the mixture at 16.7 ppmV each carbonylsulfide, ethylmercaptan,dimethylsulfide, and tetrahydrothiophene. The catalyst is similar to theone used in Example 1, except that the Pt loading is 68 g/ft³, operatedat 20,000/hr. The trap is comprised of ⅛″ pellets of 20% K/alumina,operated at a space velocity of 1510/hr. Note that the total sulfurconcentration is 150 ppmw with respect to organic carbon. This is 5 to 6times higher than the maximum concentration of sulfur in natural gas.COS and ethylmercaptan were not detected at the outlet at any time([S]<0.005 ppmV). We postulate that the thiophene is a dehydrogenationproduct of tetrahydrothiophene. No sulfur compounds are detecteddownstream of the trap at 325° C. at any time on stream to 127 hours.TABLE 1 Time on Catalyst DMS Thiophene Total S stream inlet T Outlet DMS% tHt outlet tHt % outlet Outlet Total S % (hr:min) (° C.) (ppmV)conversion ppmV) conversion (ppmV) (ppmV) conversion 24:45 275 0.05699.7 0.013 99.9 0.002 0.071 99.9 49:00 275 0.055 99.7 0.018 99.9 0.0020.075 99.9 73:18 275 0.248 98.5 0.183 98.9 0.023 0.455 99.3 97:30 2750.496 97.0 0.479 97.1 0.055 1.03 98.4 121:20  275 0.248 98.5 0.229 98.60.025 0.50 99.2 23:14 300 ND 100 ND 100 ND ND 100 47:00 300 ND 100 ND100 ND ND 100 71:46 300 0.007 100 ND 100 ND 0.007 100 96:02 300 0.01599.9 <0.010 100 ND 0.017 99.9 125:53  300 0.008 100 ND 100 0.025 0.033100

EXAMPLE 5

[0035] This example illustrates the conversion of sulfur compounds inmethane using a non-precious metal catalyst (V₂O₅). The catalyst is fromBASF (O 4-115), a Cs promoted V₂O₅ catalyst, crushed pellets operated ata space velocity=1500/hr. The trap downstream of the catalyst ismonolith supported 20% K/alumina 1 g/in³, operated at 1880/hr. The gascomposition is: 89% methane, 8.6% air, N2 (not including N₂ from air);O₂/organic C=0.020. Sulfur species and concentrations are 12.5 ppmV eachCOS, ethylmercaptan (EM), dimethylsulfide (DMS), tetrahydrothiophene(tHt). The total sulfur concentration is 112 ppmw with respect to theorganic carbon. This is 3 to 4 times higher than the maximum expectedsulfur concentration in natural gas. COS and ethylmercaptan were notdetected at the outlet at any time ([S]<0.005 ppmV). The GC analysis isperformed after 15 minutes at each temperature. We postulate that thethiophene is a dehydrogenation product of tetrahydrothiophene. Theapparent conversion at 250° C. and then increase in breakthrough Sconcentration up to 325° C. is due to adsorption on the catalyst at 250°C. followed by desorption as the temperature is increased. TABLE 2 DMSThiophene Catalyst Outlet tHt outlet outlet Total S outlet inlet Tconcentration DMS % concentration tHT % concentration concentrationTotal S % (° C.) (ppmV) conversion (ppmV) conversion (ppmV) (ppmV)conversion 250 0.49 96.1 0.003 99.9 1.65 2.16 95.7 275 2.27 81.8 0.07299.4 2.27 4.62 90.8 300 3.00 76.0 0.258 97.9 2.25 5.52 89.9 325 2.5979.3 0.094 99.2 2.04 4.72 90.6 350 1.62 87.0 <0.010 100 1.72 3.35 93.3400 0.701 94.4 ND 100 0.72 1.42 97.2 425 0.316 97.5 ND 100 0.38 0.7098.6 450 0.021 99.8 ND 100 0.14 0.16 99.7

EXAMPLE 6

[0036] This example illustrates the effect of space velocity onconversion of sulfur compounds at 275° C. in methane. For certainapplications, fuel cells need to load-follow and, thus, the natural gasflow rate will need to vary as the system adjusts power to meet loaddemands. Typical load following will require a turn down ratio of 8.

[0037] The data are average values of analyses obtained every 15 minutesfor 24 hours. The catalyst is the same as that in Example 1 (Pt/20%ZrO₂—SiO₂, 80 g/ft³ Pt). Trap: 20% K/alumina supported on a monolith at1 g/in³, 1880/hr; sulfur species and concentrations: 12.5 ppmV each COS,ethylmercaptan (EM), dimethylsulfide (DMS), tetrahydrothiophene (tHt);Gas composition: 87% methane, 0.1% to 0.2% hexane, 8.6% air, 1.5% CO₂,N₂ (not including N₂ from air); O₂/organic C=0.020. Comments: totalsulfur is 112 ppmw with respect to the organic carbon. COS andethylmercaptan were not detected time on stream ([S]<0.005 ppmV).Catalyst inlet temperature=275° C.

[0038] Sulfur breakthrough was below delectability (<5 ppbV). A slight Sbreakthrough (<30 ppbV) was observed when the total sulfur inletconcentration was raised to 50 ppmV and the space velocity was at40,000/hr. At 20,000/hr and 50 ppmV S, no breakthrough was observed.TABLE 3 DMS Space Outlet THt outlet Total S outlet velocityconcentration DMS % concentration tHt % concentration Total S % (1/hr)(ppmV) conversion (ppmV) conversion (ppmV) conversion 5,000 ND 100 ND100 100 100 10,000 ND 100 ND 100 100 100 20,000 ND 100 ND 100 100 10040,000 0.010 99.9 0.010 99.9 0.020 99.96

EXAMPLE 7

[0039] This example is directed to desulfurization of LPG. Due to thehigher sulfur content of LPG relative to the sulfur content of naturalgas and the relative ease with which LPG is oxidized, the conditions forthe catalytic desulfurization for LPG are different than for naturalgas. A mixture of propane with 5% propylene was doped with 15 ppmV eachof COS, ethylmercaptan, dimethylsulfide and tetrahydrothiophene (between65 and 80 weight part per million (ppmW) with respect topropane+propylene). The gas mixture was combined with air at a varietyof oxygen to fuel ratios ranging from 0.03 to 0.05 and the processoperated at varying inlet temperatures ranging from 275° C. to 325° C. Adownstream trap of Cs/alumina was utilized. Space velocities of20,000/hr (catalyst) and 3,000/hr (trap) were used. Table 4 sets forththe conditions and results of desulfurization. The system achievessulfur breakthrough of less than 0.6 ppmV. TABLE 4 DMS Thiosphene TotalS Catalyst Outlet tHt outlet outlet outlet inlet T O₂/C concentrationDMS % concentration tHt % concentration concentration Total S % (° C.)ratio (ppmV) Conversion (ppmV) conversion (ppmV) (ppmV) converstion 3000.03 1.4 91 0.23 98 1.6 3.2 95 300 0.04 0.23 98 0.07 99 0.02 0.32 99 3000.05 0.29 98 0.022 100 0.13 0.44 99 325 0.04 0.25 98 0.02 100 0.11 0.3899 350 0.04 0.36 98 0.01 100 0.21 0.58 99

EXAMPLE 8

[0040] Example 7 illustrating the desulfurization of LPG was repeatedexcept the inlet sulfur concentration was increased from 70 ppmW to 120ppmW (weight part per million with respect to propane+propylene; this isequal to 100 ppmV for the air-hydrocarbon mixture). An O₂/C ratio of0.04, an inlet temperature of 300° C. and a space velocity varyingbetween 10,000 to 20,000/hr. were used. Results of sulfur breakthroughare shown in Table 5. TABLE 5 DMS Thiophene Space Time on Outlet THtoutlet outlet Total S outlet velocity stream concentration DMS %concentration tHt % concentration concentration Total S % (1/hr) (hrmin) (ppmV) Conversion (ppmV) conversion (ppmV) (ppmV) Conversion 20,0000.15 3.1 88 0.7 97 1.6 5.5 78 20,000 1.20 2.8 89 0.5 98 1.0 4.3 8320,000 2.20 2.5 90 0.5 98 0.8 3.8 85 10,000 0.45 1.3 95 0.1 100 0.7 2.291 10,000 1.50 0.8 97 0.1 100 0.5 1.4 94 10,000 2.50 0.6 98 0.0 100 0.41.1 96

[0041] Although desulfurization is greater than 90% at the lower spacevelocity, the sulfur breakthrough is 1 to 2 ppmV, whereas the requiredoutlet S concentration is below 0.1 ppmV (>99.9% conversion)

EXAMPLE 9

[0042] As shown in Example 8, LPG desulfurization presents a differentchallenge than methane because of the higher concentration of sulfur inLPG and the increased reactivity of LPG to air. Accordingly, the processof Example 8 was run with a different catalyst. Thus, a catalystformulation of Pt/ZrO₂—SiO₂+Pt/CeO₂ was used under the conditions ofExample 8. TABLE 6 DMS Thiophene Space Time on Outlet tHt outlet outletTotal S outlet velocity stream concentration DMS % concentration tHt %concentration concentration Total S % (1/hr) (hr min) (ppmV) conversion(ppmV) conversion (ppmV) (ppmV) conversion 20,000 0.15 2.0 92 0.6 98 1.74.4 96 10,000 0.45 0.6 98 0.2 99 1.5 2.3 98

[0043] Although changes in selectivity are observed, there is no changein total S breakthrough.

EXAMPLE 10

[0044] Example 8 was run except the oxidation catalyst was 1:1Pt:Pd/WO₃—TiO₂ (85 g/ft³ total precious metal). The fresh catalystproduces copious quantities of H₂S at 300° C. and above. Note that witha trap formulation the includes an H₂S trap such as ZnO, the H₂S can beremoved from the process gas exit stream. When the temperature isdecreased to 275° C., the catalyst performance improves dramatically.However, with time on stream, the catalyst loses activity at 275° C.It's performance is recovered by operating at the higher temperature.The values in the table represent average catalyst out concentrations.Methylmercaptan is formed over the catalyst and is identified from itsretention time. TABLE 7 Time on Catalyst H₂S Methyl DMS Thiophene TotalS stream inlet T Outlet Mercaptan Outlet tHt outlet Outlet Outlet TotalS % (hr:min) (° C.) (ppmV) (ppmV) (ppmV) (ppmV) (ppmV) (ppmV) conversion1:00 300 1.9 0.6 0.3 0.0 0.3 3.1 97% 1:30 350 2.0 0.8 0.4 0.03 1.6 4.895% 32:00 275 0.0 0.0 0.03 0.0 0.01 0.04 100% 57:00 275 0.0 0.0 3.4 2.31.6 7.3 93% 67:00 350 ND ND ND ND ND ND 100%

EXAMPLE 11

[0045] Example 8 was run except the oxidation catalyst was Pt/TiO₂ (71g/ft³) When this catalyst is operated at 335° C. without first reducingit in H₂, the total sulfur out is initially approximately 3 ppmV, mostlyas thiophene. Over 36 hours of time on stream, the thiophene outdecreases gradually to 0.6 ppmV. When the catalyst is first reduced inH₂ before use (10% H₂ at 300° C. for two hours), the thiophene outdecreases rapidly (within 3 hours time on stream) from 2 ppmV to belowdetectibility. It gradually increases to 0.3 ppmV; the catalyst activitycan be increased by raising the inlet temperature to 350° C., where nosulfur is detected at the trap outlet.

We claim:
 1. A method for desulfurizing a hydrocarbon gas, thehydrocarbon gas comprising a hydrocarbon and a sulfur compound, themethod comprising: (a) increasing the O₂ content of the hydrocarbon gasto establish an O₂/C mole ratio in the hydrocarbon gas within the rangeof about 0.01 to less than 0.3; then (b) contacting the hydrocarbon gaswith an oxidation catalyst, wherein at least a portion of the sulfurcompound is oxidized to SO_(x); then (c) contacting the SO_(x) with anadsorbent capable of adsorbing SO_(x), wherein at least a portion of theSO_(x) is adsorbed on the adsorbent.
 2. The method of claim 1 whereinthe oxidation catalyst is sulfur tolerant and is supported by a sulfurtolerant support material.
 3. The method of claim 1 wherein theoxidation catalyst comprises a precious metal, a vanadium oxide, acerium oxide, or a base metal oxide.
 4. The method of claim 1 whereinthe oxidation catalyst comprises a platinum-containing material.
 5. Themethod of claim 1 wherein the step of increasing the O₂ content of thehydrocarbon gas is performed to establish an O₂/C mole ratio within therange of about 0.01 to 0.08.
 6. The method of claim 1 wherein the stepof increasing the O₂ content of the hydrocarbon gas is performed toestablish an O₂/C mole ratio within the range of about 0.01 to 0.04. 7.The method of claim 1 wherein the hydrocarbon gas comprises methane. 8.The method of claim 1 wherein the hydrocarbon gas comprises liquefiedpetroleum gas.
 9. The method of claim 1 wherein the hydrocarbon gas iswithin the temperature range of about 200-600° C. when contacting theoxidation catalyst.
 10. The method of claim 1 wherein the hydrocarbongas is produced by vaporizing a liquid hydrocarbon.
 11. The method ofclaim 1, wherein the adsorbent capable of adsorbing SO_(x) is a metaloxide selected from the group consisting of an alkali metal oxide, analkali earth metal oxide and a base metal oxide.
 12. The method of claim1, further comprising the steps of converting at least a portion of thesulfur compound to H₂S, then contacting the H₂S with an adsorbentcapable of adsorbing H₂S.
 13. The method of claim 12 wherein theadsorbent capable of adsorbing H₂S is zinc oxide.
 14. The method ofclaim 1, further comprising the step of reforming the hydrocarbon gas toproduce H₂ after step (c).
 15. The method of claim 14, furthercomprising the step of utilizing the H₂ in a fuel cell to produceelectricity.
 16. A method for desulfurizing a hydrocarbon gas, thehydrocarbon gas comprising a hydrocarbon, a sulfur compound and O₂,wherein the hydrocarbon gas has an O₂/C mole ratio within a range thatenables catalytic oxidation of the sulfur compound to SO_(x), the methodcomprising: a) contacting the hydrocarbon gas with an oxidationcatalyst, wherein at least a portion of the sulfur compound isselectively oxidized to SO_(x); then b) contacting the SO_(x) with anadsorbent capable of adsorbing SO_(x), wherein at least a portion of theSO_(x) is adsorbed on the adsorbent.
 17. The method of claim 16 whereinthe oxidation catalyst is sulfur tolerant and is supported by a sulfurtolerant material.
 18. The method of claim 16 wherein the oxidationcatalyst comprises a precious metal, a vanadium oxide, a cerium oxide ora base metal oxide.
 19. The method of claim 16 wherein the oxidationcatalyst comprises a platinum-containing material.
 20. The method ofclaim 16 wherein the hydrocarbon gas comprises methane.
 21. The methodof claim 16 wherein the hydrocarbon gas comprises liquefied petroleumgas.
 22. The method of claim 16 wherein the O₂/C mole ratio of thehydrocarbon gas is within the range of about 0.01 to less than 0.3. 23.The method of claim 16 wherein the O₂/C mole ratio of the hydrocarbongas is within the range of about 0.01 to 0.08.
 24. The method of claim16 wherein the hydrocarbon gas is within the temperature range of about200-600° C. when contacting the oxidation catalyst.
 25. The method ofclaim 16 wherein the hydrocarbon gas is produced by vaporizing a liquidhydrocarbon.
 26. The method of claim 16 wherein the adsorbent capable ofadsorbing SO_(x) is a metal oxide selected from the group consisting ofan alkali metal oxide, an alkali earth metal oxide and a base metaloxide.
 27. The method of claim 16, further comprising the steps ofconverting at least a portion of the sulfur compound to H₂S, thencontacting the H₂S with an adsorbent capable of adsorbing H₂S.
 28. Themethod of claim 27 wherein the adsorbent capable of adsorbing H₂S iszinc oxide.
 29. The method of claim 16, further comprising the step ofreforming the hydrocarbon gas to produce H₂ after step (b).
 30. Themethod of claim 35, further comprising the step of utilizing the H₂ in afuel cell.